Illumination system for a microlithographic projection exposure apparatus

ABSTRACT

Illumination systems for microlithographic projection exposure apparatus, as well as related systems, components and methods are disclosed. In some embodiments, an illumination system includes one or more scattering structures and an optical integrator that produces a plurality of secondary light sources.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation U.S. patent application Ser.No. 12/190,308 filed Aug. 12, 2008, which is a continuation ofinternational patent application serial number PCT/EP2007/001267 filedFeb. 14, 2007, which claims benefit of U.S. provisional patentapplication Ser. No. 60/774,850 filed Feb. 17, 2006. The full disclosureof each prior application is incorporated herein by reference.

FIELD

The disclosure relates generally to illumination systems formicrolithographic projection exposure apparatus, as well as relatedsystems, components and methods. In some embodiments, an illuminationsystem includes one or more scattering structures and an opticalintegrator that produces a plurality of secondary light sources.

BACKGROUND

Microlithography (also called photolithography or simply lithography) isa technology for the fabrication of integrated circuits, liquid crystaldisplays and other microstructured devices. The process ofmicrolithography, in conjunction with the process of etching, is used topattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)light. Next, the wafer with the photoresist on top is exposed toprojection light through a mask in a projection exposure apparatus. Themask contains a circuit pattern to be projected onto the photoresist.After exposure the photoresist is developed to produce an imagecorresponding to the circuit pattern contained in the mask. Then an etchprocess transfers the circuit pattern into the thin film stacks on thewafer. Finally, the photoresist is removed. Repetition of this processwith different masks results in a multilayered microstructuredcomponent.

A projection exposure apparatus typically includes an illuminationsystem, a mask stage for a aligning the mask, a projection lens and awafer alignment stage for aligning the wafer coated with thephotoresist. The illumination system illuminates a field on the maskthat often has the shape of an (elongated) rectangle or a ring segment.

In current projection exposure apparatus a distinction can be madebetween two different types of apparatus. In one type each targetportion on the wafer is irradiated by exposing the entire mask patternonto the target portion in one go; such an apparatus is commonlyreferred to as a wafer stepper. In the other type of apparatus, which iscommonly referred to as a step-and-scan apparatus or scanner, eachtarget portion is irradiated by progressively scanning the mask patternunder the projection light beam in a given reference direction whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection. The ratio of the velocity of the wafer and the velocity ofthe mask is equal to the magnification of the projection lens, which isusually smaller than 1, for example 1:4.

It is to be understood that the term “mask” (or reticle) is to beinterpreted broadly as a patterning device. Commonly used masks containtransmissive or reflective patterns and may be of the binary,alternating phase-shift, attenuated phase-shift or various hybrid masktype, for example. However, there are also active masks, e.g. masksrealized as a programmable mirror array. An example of such a device isa matrix-addressable surface having a viscoelastic control layer and areflective surface. More information on such mirror arrays can begleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No.5,523,193. Also programmable LCD arrays may be used as active masks, asis described in U.S. Pat. No. 5,229,872. For the sake of simplicity, therest of this text may specifically relate to apparatus including a maskand a mask stage; however, the general principles discussed in suchapparatus should be seen in the broader context of the patterningdevices as noted above.

The angular distribution of the projection light impinging on the maskis usually adapted to the kind of pattern to be projected onto thephotoresist. For example, relatively large sized features may involve adifferent angular distribution than small sized features. Commonly usedangular distributions of projection light are referred to asconventional, annular, dipole and quadrupole illumination settings.These terms refer to the irradiance distribution in a pupil plane of theillumination system. With an annular illumination setting, for example,only an annular region is illuminated in the pupil plane, and thus thereis only a small range of angles present in the angular distribution ofthe projection light so that all light beams impinge obliquely withsimilar angles onto the mask.

In illumination systems designed for wavelengths below 200 nm, lasersare typically used as light sources. The projection light bundle emittedby a laser typically has a small cross section and a low divergence, andtherefore also the geometrical optical flux is small. The geometricaloptical flux, which is also referred to as the Lagrange invariant, is aquantity that is, at least for certain special configurations,proportional to the product of maximum light angle and size of theilluminated field.

SUMMARY

In some embodiments, the disclosure provides an illumination system fora microlithographic projection exposure apparatus that makes it possibleto achieve a desired irradiance and angular distribution in a maskplane. A uniform or desired nonuniform irradiance distribution can beachieved that is substantially independent from the illuminationsetting.

In certain embodiments, an illumination system includes a light sourceand an optical integrator. The optical integrator includes first opticalsubelements and produces a plurality of secondary light sources eachemitting a light bundle. The condenser effects a superposition of thelight bundles in a mask plane. At least one scattering structure isprovided that includes a plurality of second optical subelements thatare arranged in front of or behind the secondary light sources. Thefirst and second optical subelements are configured such that opticalsubelements illuminated with identical irradiance distributions areseparated by more than 5 mm.

This configuration can reduce undesired interactions between the atleast one scattering structure and the optical integrator that mayresult in fluctuations in the irradiance distribution obtained in a maskplane of the illumination system.

In some embodiments, an illumination system includes an opticalintegrator and at least one scattering structure. The latter has aplurality of subelements that produce rectangular angular distributionshaving different angular widths.

Such a scattering structure can produce a Gaussian angular distributionhaving a half value width that may be easily defined by selectingsubstructures producing angular distributions having appropriate angularwidths. Such a scattering structure may be advantageously arrangedbetween the optical integrator and a condenser that superposes secondarylight sources produced by the optical integrator.

In some embodiments, an illumination system includes a light source andan optical integrator producing secondary light sources. A firstscattering structure produces a substantially rectangular angulardistribution in one direction. A second scattering structure produces asubstantially Gaussian angular distribution in two orthogonaldirections. Such an illumination system can be particularly suitable forproducing slit-shaped illuminated fields that are involved forprojection exposure apparatus of the step-and-scan type.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present disclosure may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 is a perspective and considerably simplified view of a projectionexposure apparatus;

FIG. 2 is a meridional section through an illumination system containedin the projection exposure apparatus shown in FIG. 1;

FIG. 3 is a perspective view of an optical integrator and two scatteringplates contained in the illumination system shown in FIG. 2;

FIG. 4 is a section through the optical integrator shown in FIG. 3parallel to an X-Z plane;

FIG. 5 is a section through the optical integrator shown in FIG. 3parallel to a Y-Z plane;

FIG. 6 is a schematic representation of secondary light sources producedby the optical integrator;

FIG. 7 is a side view of the optical integrator shown in FIG. 3;

FIGS. 8 to 12 show optical integrators in representations similar toFIG. 7;

FIG. 13 is a section through the optical integrator and the firstscattering plate parallel to an X-Z plane;

FIG. 14 is a graph illustrating the angular distribution produced by thefirst scattering plate shown in FIG. 13;

FIG. 15 is a schematic representation of the secondary light sourcessimilar to FIG. 6 as it is produced when using the first scatteringplate shown in FIG. 13;

FIG. 16 is an enlarged cutout of FIG. 13;

FIG. 17 is an illustration similar to FIG. 16 in which the firstscattering plate is arranged between two integrator members;

FIG. 18 is a graph schematically illustrating a plurality of Talbotinterference patterns;

FIG. 19 is a still further enlarged cutout similar to FIG. 16;

FIG. 20 is a top view of two grids for illustrating the merits of anembodiment;

FIG. 21 is a perspective view of a first scattering plate;

FIG. 22 is a section through the first scattering plate shown in FIG. 21in an X-Z plane;

FIGS. 23 to 28 show various embodiments for the first scattering platein a side views similar to FIG. 22;

FIG. 29 is a top view of an embodiment having microlenses with varyingwidths;

FIGS. 30 to 32 show embodiments of a first scattering plate in sectionalviews similar to FIG. 22;

FIG. 33 is a perspective view of a first scattering plate according toan embodiment;

FIG. 34 is a section through the first scattering plate shown in FIG. 33along line XXXIV-XXXIV;

FIG. 35 is a top view of a first scattering plate including a pluralityof rotationally symmetric microlenses;

FIG. 36 is a section through the first scattering plate shown in FIG. 35along line XXXVI-XXXVI;

FIG. 37 is a top view on a diffractive cell for use in the firstscattering plate;

FIGS. 38 a and 38 b are schematic top views on diffractive cells formingFresnel lenses having different curvatures;

FIGS. 39 a and 39 b are top views on diffractive cells forming cylinderlenses having different lateral positions;

FIGS. 40 to 43 are schematic top views on cell arrangements for thefirst scattering plate;

FIG. 44 is a graph illustrating the angular distribution produced by thesecond scattering plate;

FIG. 45 is a graph illustrating the formation of a Gaussian angulardistribution by superposing rectangular distributions of differentwidths;

FIGS. 46 and 47 are schematic views of a part of the optical integratorand the condenser contained in the illumination system shown in FIG. 2without and with the second scattering plate, respectively;

FIG. 48 is a graph showing an exemplary intensity distribution in thepupil plane of the illumination system shown in FIG. 2, but without thesecond scattering plate;

FIG. 49 shows the intensity distribution of FIG. 48, but with the secondscattering plate inserted.

DETAILED DESCRIPTION

1. General Structure of Projection Exposure Apparatus

FIG. 1 shows a perspective and highly simplified view of a projectionexposure apparatus 10 that includes an illumination system 12 forproducing a projection light bundle. The projection light bundleilluminates a field 14 on a mask 16 containing minute structures 18. Theilluminated field 14 has approximately the shape of a ring segment.However, other, for example rectangular, shapes of the illuminated field14 are contemplated as well.

A projection objective 20 images the structures 18 within theilluminated field 14 onto a light sensitive layer 22, for example aphotoresist, which is deposited on a substrate 24. The substrate 24,which may formed by a silicon wafer, is arranged on a wafer stage (notshown) such that a top surface of the light sensitive layer 22 isprecisely located in an image plane of the projection objective 20. Themask 16 is positioned by a mask stage (not shown) in an object plane ofthe projection objective 20. Since the latter has a magnification ofless than 1, a minified image 14′ of the structures 18 within theilluminated field 14 is projected onto the light sensitive layer 22.

During the projection, the mask 16 and the substrate 24 move along ascan direction which coincides with the Y direction. Thus theilluminated field 14 scans over the mask 16 so that structured areaslarger than the illuminated field 14 can be continuously projected. Sucha type of projection exposure apparatus is often referred to as“step-and-scan tool” or simply a “scanner”. The ratio between thevelocities of the mask 16 and the substrate 24 is equal to themagnification of the projection objective 20. If the projectionobjective 20 inverts the image, the mask 16 and the substrate 24 move inopposite directions, as this is indicated in FIG. 1 by arrows A1 and A2.However, the present disclosure may also be used in stepper tools inwhich the mask 16 and the substrate 24 do not move during projection.

The illuminated field 14 is not centered with respect to an optical axis26 of the projection objective 20. Such an off-axis illuminated field 14may be involved with certain types of projection objectives 20, forexample objectives that contain one or more truncated mirrors. As amatter of course, the present disclosure may also be employed inillumination systems with a centered illuminated field.

2. General Structure of Illumination System

FIG. 2 is a more detailed meridional section through the illuminationsystem 12 shown in FIG. 1. For the sake of clarity, the illustration ofFIG. 2 is also considerably simplified and not to scale. Thisparticularly implies that different optical units are represented byvery few optical elements only. In reality, these units may includesignificantly more lenses and other optical elements.

The illumination system 12 includes a housing 28 and a light source thatcan be an excimer laser 30. The excimer laser 30 emits projection lightthat has a wavelength of about 193 nm.

Other types of light sources and other wavelengths, for example 248 nmor 157 nm, are also contemplated.

The projection light emitted by the excimer laser 30 enters a beamexpansion unit 32 in which the light bundle is expanded. After passingthrough the beam expansion unit 32, the projection light impinges on afirst optical raster element 34. The first optical raster element 34 isreceived in a first exchange holder 36 so that it can easily be removedor replaced by other optical raster elements having differentproperties. The first optical raster element 34 includes one or morediffraction gratings that deflect each incident ray such that adivergence is introduced. This means that at each location on theoptical raster element 34, light is diffracted within a certain range ofangles. This range may extend, for example, from −3° to +3°. In FIG. 2this is schematically represented for two off-axis rays 38 a, 38 b thatare split into a plurality of diverging rays 38. The first opticalraster element 34 thus slightly increases the geometrical optical fluxand modifies the local irradiance distribution in a subsequent pupilplane. Other kinds of optical raster elements, for example microlensarrays or an array of phase step or grey-tone Fresnel lenses, may beused instead or additionally.

The first optical raster element 34 is positioned in an object plane 42of an objective 44 that includes a zoom lens group 46 and a pair 48 ofaxicon elements 50, 52 having opposing conical faces. If both axiconelements 50, 52 are in contact, as is shown in FIG. 2, the axicon pair48 has the effect of a plate having parallel plane surfaces. If bothelements 50, 52 are moved apart, the spacing between the axicon elements50, 52 results in a shift of light energy radially outward. Since axiconelements are known as such in the art, these will not be explained herein further detail.

Reference numeral 54 denotes an exit pupil plane of the objective 44. Anoptical integrator, which is denoted in its entirety by 56 and will beexplained in more detail below with reference to FIGS. 3 to 5, ispositioned in or in close proximity to the exit pupil plane 54 of theobjective 44. The optical integrator 56, which is received in anexchange holder 57, modifies the angular distribution in the pupil plane54. Since all light rays passing the pupil plane under the same angleconverge to a single point in a subsequent Fourier related field plane,the angular distribution in the pupil plane 54 directly translates intoan irradiance distribution in such a field plane. Thus the design of theoptical integrator 56 has a strong influence on the irradiancedistribution and the geometry of the illuminated field 14 on the mask16. If the illuminated field 14 has the shape of a curved slit as isshown in FIG. 1, the exit side numerical aperture of the opticalintegrator 56 may, as a non-limiting example, be in the range from 0.28to 0.35 in the X direction and in the range from 0.07 to 0.09 in the Ydirection. The optical integrator produces a plurality of secondarylight sources that each emits a light bundle.

In front of and behind the optical integrator 56 scattering plates arearranged that are denoted by 58 and 60, respectively, and whosestructure and function will be elucidated further below.

The projection light emerging from the secondary light sources enters acondenser 62 that is represented in FIG, 2 by a single lens element forthe sake of simplicity. The entrance pupil plane of the condenser 62coincides with the exit pupil plane 54 of the objective 44. Thecondenser 62 superposes the light bundles emitted by the secondary lightsources in a field stop plane 64 of the condenser 62 in which a fieldstop 66 is positioned. A field stop objective 68 images the field stop66 onto a mask plane 70 in which the mask 16 is positioned. The fieldstop 66 ensures sharp edges of the illuminated field 14 at least for theshort lateral sides extending along the Y direction. The field stop maybe realized, for example, by two orthogonal pairs of blades. However,the (additional) use of an adjustable stop device as is disclosed in EP0 952 491 A2 is also possible.

3. Optical Integrator

In the following the general structure and the function of the opticalintegrator 56 used in the illumination system 12 will be described inmore detail with reference to FIGS. 3 to 5. FIG. 3 shows the opticalintegrator 56 and the scattering plates in a perspective view, and FIGS.4 and 5 show the optical integrator 56 in sections parallel to the X-Zplane and the Y-Z plane, respectively.

3.1 General Structure of Optical Integrator

The optical integrator 56, which is known as such from InternationalApplication WO 2005/078522 A2 assigned to the applicant, includes afirst integrator member 561 and a second integrator member 562. Thefirst integrator member 561 includes a first array of cylindricalmicrolenses 561Y having parallel longitudinal axes that are alignedalong the X direction. Thus the first microlenses 561Y have a positiverefractive power only in the Y direction with a back focal length f₁.

The first integrator member 561 further includes an array of secondcylindrical microlenses 561X that have parallel longitudinal axesaligned along the Y direction. The second microlenses 561X have apositive refractive power only in the X direction with a back focallength f₂<f₁.

The second integrator member 562 is an identical copy of the firstintegrator member 561, but is mounted after a rotation by 180° aroundeither the X or Y axis. Thus third microlenses 562X are facing thesecond microlenses 561X, and fourth microlenses 562Y are facing thesecond scattering plate 60.

As can be seen in FIGS. 4 and 5, the focal lengths fi and Ï₂ and thedistance between the integrator members 561, 562 are selected such thatfocal lines produced by the second microlenses 561X are located on thevertices of the third microlenses 562X. Since the third microlenses 562Xhave the same focal length f₂ as the second microlenses 561X, thisimplies that the focal lines of the third microlenses 562X are locatedon the vertices of the second microlenses 561X. In FIG. 4 this mutualcorrespondence is illustrated by rays 81 drawn in broken lines.

From FIG. 5 it becomes clear that the same conditions also apply to thefirst and fourth microlenses 561Y, 562Y, respectively, having equalfocal lengths fi. Since the focal lines are located on the curvedsurfaces of the third and fourth microlenses 562X, 562Y and not insidethese microlenses, very high light intensities that could destroy thematerial of the microlenses or a substrate supporting the latter cannotoccur.

In FIGS. 3 to 5 the microlenses 561Y, 561X, 562Y, 562X are representedas subelements that have plane back surfaces and are attached to planeback surfaces of the adjacent microlenses. However, the integratormembers 561, 562 will usually not be assembled from separatesubelements, but will be manufactured in a more efficient way, forexample by molding or by machining a substrate having originally planeand parallel surfaces. The technology used for manufacturing theintegrator members 561, 562 also depends on the pitch of the microlenses561X, 561Y, 562X, 562Y, which may be in the range of one or severalmillimeters. However, for reasons explained further below with referenceto FIG. 6, it may be desirable to have a pitch below 1 mm, for example500 μm. The pitch generally denotes the width of a microlens along thedirection in which it has a refractive power. In the case of a cylinderlens, the pitch is equal to the dimension of the microlens perpendicularto its longitudinal extent.

From these remarks it becomes clear that the illustrations of FIGS. 3 to5 are greatly simplified and not to scale. For example, if theintegrator members 561, 562 have lateral dimensions of 25 mm and thepitch of the microlenses equals 500 μm, each array will be made up of 50microlenses. It is to be understood, however, that the pitch and thenumber of microlenses 561X, 561X having a refractive power in the Xdirection do not have to be equal to the pitch and number of themicrolenses 561Y, 561Y having a refractive power in the Y direction.

In order to keep transmission losses small, the integrator members 561,562 shown in FIGS. 3 to 5 are made of CaF₂, which has, for thecontemplated wavelength of 193 nm, a higher transmission than fusedsilica (SiO₂)· If wavelengths below 193 nm are used, fused silica isalmost opaque so that CaF₂ or a similar fluorite material should beused.

Since CaF₂ is a brittle material that is difficult to machine, thethickness of the substrate should exceed approximately 2 mm. If thepitch of the microlenses 561Y, 561X, 562Y, 562X shall be kept below 1 mmand the refractive power of the microlenses 561X, 562X shall besufficient to produce a numerical aperture NA in excess of about 0.2, asequence of microlens arrays having refractive power in the Y direction,the X direction, the X direction and the Y direction, as is the case inFIGS. 3 to 5, is the only possible way. With other configurations, forexample the configuration disclosed in U.S. Pat. No. 4,682,885 A, allaforementioned conditions could not be simultaneously fulfilled.

It is to be understood that the configuration of the optical integratormay be varied in various kinds For example, the microlenses 561Y, 561X,562Y, 562X may be concavely or aspherically shaped. Asphericalmicrolenses may be used, for example, for achieving special non-uniformirradiance distributions in the mask plane 70, for example distributionsin which the irradiance at the edges is slightly higher than in thecentre. More particularly, the irradiance at the edges of theilluminated field may be at least 0.5% to 0.8% higher than in the centerof the illuminated field. Such a nonuniform irradiance distributionprovides compensation if light emerging from edges experiences higherlosses in the projection objective that light emerging from the center.

Furthermore it may be advantageous if adjacent microlenses aredifferent, as is described in US Pat. Appl. No. 2005/0018294 A1 that hasbeen mentioned further above. Instead of using microlenses formed asconvexly curved cylinder lenses, other configurations may becontemplated as well, for example embodiments including an array ofrotationally symmetrical microlenses or microlenses that are obtained bycrossing two arrays of microlenses having a cylindrical or toric shape.Similar configurations are shown in FIGS. 33 to 36 below for the firstscattering plate 58.

Apart from that it is also contemplated to use diffractive opticalelements instead of the microlenses for achieving refractive power inthe X and/or Y direction.

3.2 Function of Optical Integrator

In the following the function of the optical integrator 56 will bebriefly explained.

If this projection light beam is perfectly collimated so that all lightrays are parallel to the Z axis, the second integrator member 562 may bedispensed with. The first integrator member 561 then alone produces aplurality of secondary light sources. A light beam falling on the firstintegrator member 561 is diverted in the Y direction and, to a largerextent due to the larger refractive power of the microlenses 561X, 562X,in the X direction. Thus each secondary light source produces ananamorphic angular distribution.

However, the light impinging on the optical integrator 56 is usually notperfectly collimated, but has a small divergence. Without the secondintegrator member 562, this divergence would cause a parallax that mayresult in an undesired shift of the illuminated field 14 on the mask 16.

The second integrator member 562 ensures that a parallax does not occureven if the impinging light is not perfectly collimated. As becomesclear from FIGS. 4 and 5, the second integrator member 562 has littleeffect on parallel rays impinging on the optical integrator 56, sincethe focal lines are located on the vertices of the microlenses 562X,562Y of the second integrator member 562. For rays hitting the opticalintegrator 56 not parallel to the Z axis but under a certain angle, themicrolenses 562X, 562Y of the second integrator member 562 ensure thatthese rays are transformed into telecentric bundles.

FIG. 6 shows in a schematic representation secondary light sources 82produced in the second integrator member 562 as viewed from the maskside along the optical axis 26. Only those secondary light sources 82contribute to the illumination of the mask 16 that are actually exposedto the projection light beam impinging on the optical integrator 56. Theshape of this projection light beam depends on the illumination setting.For example, in a conventional illumination setting with maximumcoherence parameter σ the optical elements preceding the opticalintegrator 56 in the path of light produce a projection light beamhaving a circular cross-section that is indicated in FIG. 6 by 80.

Ideally all secondary light sources 82 produce light bundles having thesame angular (anamorphic) distribution. In a subsequent Fouriertransformed field plane, namely the field stop plane 64 or the maskplane 70 conjugated thereto, these angular distributions translate intoirradiance distributions. If the angular distribution produces by thesecondary light sources is a rectangular distribution in which allangles occur with the same irradiance, a perfectly uniform irradiancedistribution is obtained in the mask plane 70.

However, due to manufacturing tolerances and other reasons, the angulardistributions produced by the secondary light sources 82 are usually notperfectly identical. Nevertheless a homogeneous irradiance distributionwill be obtained in the mask plane 70 if the angular distributions ofthe secondary light sources 82 statistically vary. If the number ofsecondary light sources 82 is sufficiently large, all non-homogeneousirradiance distributions produced by each secondary light source 82superpose in the mask plane 70, and irradiance variations will becancelled out due to the averaging effect achieved by the superpositionof all irradiance distributions produced by the secondary light sources82.

From this it becomes clear that it is advantageous to have a largenumber of secondary light sources because this improves theaforementioned averaging effect. Another advantage of having a largenumber of secondary light source, and thus a small pitch of themicrolenses 561X, 561Y, 562X, 562Y, is that the illumination system 12,and in particular the first optical raster element 34, the zoomobjective 46 and the pair 48 of axicon elements 50, 52, makes itpossible to produce a wide variety of different illumination settings.This involves that the cross-section of a light beam impinging on theoptical integrator 56 may differ significantly.

If the number of secondary light sources is small and there aresignificant gaps between the secondary light sources, there may besymmetric illumination settings wherein the “active”, i.e. illuminated,secondary light sources are not symmetrically distributed over the clearaperture 80. This may cause undesired asymmetric angular distributionsof the projection light impinging on the mask 16. In contrast, if thereis a large number of small and densely arranged secondary light sources,the probability of significant asymmetries of this kind is reduced.

3.3 Alternative Arrangements of Optical Integrator

In the following various alternative arrangements of the microlensarrays and the scattering plates 58, 60 will be described with referenceto FIGS. 7 to 12. Between subsequent embodiments, corresponding partsare denoted by reference numerals increased by 1000 and will not alwaysbe referred to again.

FIG. 7 shows the optical integrator 56 and the scattering plates 58, 60of FIG. 3 in a side view. In this and the similar representations ofFIGS. 8 to 12 cylindrical microlenses extending along the X directionare hatched with vertical lines, whereas cylindrical microlensesextending in the Y direction are hatched with horizontal lines.

FIG. 7 differs from FIGS. 3 to 5 in that the first integrator member 561may be adjusted with the help of an adjusting device 561A. The adjustingdevice 561A is only schematically indicated and may be driven using amicrometer screw or a piezo element, for example. Via the adjustingdevice 561A it is possible to adjust the distance between the integratormembers 561, 562 along the Z axis. The focal lines produced by the firstmicrolenses 561Y can then be positioned exactly on the vertices of thefourth microlenses 562Y. Alternatively, the focal lines produced by thesecond microlenses 561X can be positioned exactly on the vertices of thethird microlenses 562X.

FIG. 8 shows the last two microlens arrays are arranged in reversedorder. Thus third microlenses 1562Y having refractive power in the Ydirection are now facing second microlenses 1561X of the firstintegrator member 1561. In order to maintain the aforementioned focalline property, which is indicated in FIGS. 7 to 12 by arrows havingdifferent lengths and style, the first integrator member 1561 has to bequite thick, whereas the second integrator member 1562 has to be quitethin. For facilitating the manufacture of the integrator members 1561,1562, the support for the microlenses should therefore be made of amaterial that is less brittle than CaF₂, for example fused silica.

FIG. 8 has a second adjusting device 1562A for the second integratormember 1562. Having independent adjusting devices for each integratormember 1561, 1562 makes it possible not only to adjust the distancespacing between the integrator members 1561, 1562, but also the distanceof the integrator members 1561, 1562 with respect to the scatteringplates 58, 60.

FIG. 9 differs from FIG. 7 only in that second microlenses 2561X areformed not on the same support as first microlenses 2561Y, but on aseparate support that may also be made of CaF₂, for example. With thehelp of an adjustment device 2562A, the second microlenses 2561X may beadjusted along the Z direction independently from first the microlenses2561Y and third and fourth microlenses 2563X, 2563Y arranged on a commonthird support. Having three integrator members attached to separateadjusting devices 2561A, 2562A, 2563A makes it possible to adjust thedistances between each pair of corresponding microlens arraysindependently. Having three adjustable microlens arrays can therefore anoptimum solution if full adjustability is desired with as littlecomplexity as possible.

FIG. 10 differs from FIG. 9 in that the first scattering plate 58 is nowarranged between first microlenses 3561Y and 3562X. The scattering plate58 may simply be shifted as a whole, or its function may be achieved, asshown in FIG. 10, by forming a scattering structure 58′ on the otherside of a substrate on which the second microlenses 3562X are formed.

FIG. 11 shows an optical integrator having three integrator members thatmay be independently adjusted with the help of adjusting devices 4561A,4562A and 4563A. First and second microlenses 4561Y and 4562Y having arefractive power in the Y direction are arranged on different substratesso that they are facing each other. Third and fourth microlenses 4562Xand 4563X are facing each other, too. The second microlenses 4562Y andthe third microlenses 4562X are formed on the same substrate and maytherefore be commonly adjusted by the second adjusting device 4562A.Here the distances between the integrator members 4561, 4562 and 4563are selected such that the back focal lines of the second microlenses4562Y and the fourth microlenses 4563X are positioned in a common plane4587. This plane 4587 may then be Fourier transformed into the fieldstop plane 64 by the condenser 62. As a matter of course, a similarproperty may also be achieved in FIG. 8, because the fourth microlenses1562X having the shorter focal lengths are positioned behind the thirdmicrolenses 1562Y having the longer focal lengths.

FIG. 12 is similar to in FIG. 7. However, the first scattering plate 58is not positioned in front of the optical integrator, but between theintegrator members 5561, 5562.

4. First Scattering Plate

In the following the general function and various embodiments for thefirst scattering 58 plate will be explained in more detail. As a matterof course, this also holds true for the other embodiments of the opticalintegrator 56 shown in FIGS. 7 to 12. However, for the sake ofsimplicity the following remarks will refer only to FIGS. 3 to 6.

4.1 General Function of First Scattering Plate

One function of the first scattering plate 58 is to adapt the smallgeometrical optical flux produced by the light source 30 (and possiblyby the first optical raster element 34, if inserted) to the highergeometrical optical flux of the optical integrator 56. This has theadvantageous effect of preventing high light intensities within thesecond integrator member 562 that could destroy the material of whichthe optical integrator 56 is made.

In this respect it should be noted that, although the focal linesproduced by the first and second microlenses 561Y, 561X are located onthe vertices of the fourth microlenses 562Y and third microlenses 562X,respectively, and not inside these microlenses, high light intensitiesmay nevertheless occur in the immediate vicinity of the vertices. Thefirst scattering plate 58 ensures that the projection light impinging onthe first integrating member has a sufficient divergence such thatnarrow focal lines, as are shown in FIGS. 4 and 5, are avoided. Ideally,the first scattering plate 58 is adapted to the second microlenses 561Xsuch that light passing a microlens 561X completely illuminates a thirdcorresponding microlens 562X in the second integrator member 562.Mathematically, this condition may be described by

0.5·(D/2f ₂−NA_(pre))<NA_(SC)<(D/2f ₂−NA_(pre))

where D is the diameter of the array of second microlenses 561X, NAsc isthe numerical aperture of the first scattering plate 58 and NA_(pre) isthe numerical aperture of the light impinging thereon.

This is shown in FIG. 13 in a representation similar to FIG. 4. In thisillustration it is indicated that projection light 84 impinging on thescattering element 58 has a low divergence. Without the first scatteringplate 58, the projection light 84 would be focused by the secondmicrolenses 561X on the vertices of the third microlenses 562X, as isshown in FIG. 4. The first scattering plate 58, however, increases thedivergence to such an extent that light, which passes through a secondmicrolens 561X, is not focused on the vertex of the corresponding thirdmicrolens 562X, but is distributed over its entire curved surface.

In projection exposure apparatus of the scanner type, the field 14illuminated on the mask 16 has a high aspect ratio. This means that thedimension of the field 14 along the scan direction (the Y direction) ismuch shorter than its dimension along the X direction. In the absence ofany stops that block out projection light, the aspect ration of theilluminated field would be determined by the maximum angles of theprojection light in the X and Y direction occurring in the pupil plane54.

This means that the projection light passing the pupil plane 54 shouldhave a small divergence in the Y direction and a larger divergence inthe X direction. This is the reason why the first focal length fi of thefirst and fourth microlenses 561Y, 562Y is larger than the focal lengthf₂ of the second and third microlenses 561X, 562X. Since the firstscattering plate 58 additionally increases the divergence, as is shownin FIG. 13, the amount of the divergence introduced by the firstscattering plate 58 in the X and Y directions should be carefullyadapted to the divergence introduced by the optical integrator 56. Forthat reason the divergence introduced by the first scattering plate 58is generally higher in the X direction than in the Y direction. In thecase of illuminated fields 14 having a very large aspect ratio, thefirst scattering plate may even be designed such that it increases thedivergence only in the X direction but not (or not substantially) in theY direction.

FIG. 14 shows a graph illustrating the angular distribution ofprojection light after having traversed the first scattering plate 58.As can be seen in FIG. 14, the angular distribution in the X directionis substantially rectangular. This means that all angles having anabsolute value smaller than a maximum angle α_(max) occur with the sameintensity. However, even with an ideal first scattering plate 58 such arectangular angular distribution cannot be obtained because the lightsource 30, which is usually realized as a laser, produces projectionlight that has itself an angular distribution. In the case of a laserlight source, this distribution has a Gaussian shape which results insmooth slopes at the maximum angles ±α_(max). For the same reason thelight has a small angular distribution in the Y direction even if thefirst scattering plate 58 does not as such increase the divergence inthe Y direction. In this case the angular distribution in the Ydirection is given by the Gaussian angular distribution produced by thelaser light source.

If the first scattering plate 58 has a strongly anamorphic effect, i.e.increasing the divergence in the X and Y direction to different extents,it should be positioned rather close in front of the optical integrator56. In some embodiments, the distance z between the first scatteringplates 58 and the optical integrator is below 20 mm.

Another important function of the first scattering plate 58 is toincrease the size of the secondary light sources 82. This increase isdue to the fact that the third microlenses 562X are now more completelyilluminated so that the size of the secondary light sources in the Xdirection increases. In the Y direction the size of the secondary lightsources increases only if the first scattering plate 58 increases thedivergence also in the Y direction.

FIG. 15 shows, in a representation similar to FIG. 6, the secondarylight sources 82′ that are obtained with a first scattering plate 58producing an anamorphic angular distribution. In comparison with thesecondary light sources 82 shown in FIG. 6, the secondary light sources82′ are now broadened in the X direction so that the gaps extendingalong the Y direction between adjacent secondary light sources almostvanish. The more completely the secondary light sources 82′ fill thepupil, the more continuous are the angular distributions obtained in themask plane 70. The second scattering plate 60 which will be describedfurther below, may further enhance the filling factor of the pupil.

In section 4.3 various embodiments of first scattering plates 58 will bedescribed which ensure that the first and second microlenses 561Y, 561Xof the first integrator member 561 are traversed by projection lighthaving random or randomized angular distributions. As a result, thesecondary light sources 82′ shown in FIG. 15 produce, at least ingeneral, different angular distributions and thus different irradiancedistributions in the mask plane 70. However, due to the random orrandomized angular distributions produced by the first scattering plate58, these irradiance distributions in the mask plane 70 also varystatistically. The superposition of a large number of statisticallyvarying irradiance distributions in the mask plane 70 results in anoverall irradiance distribution that is almost perfectly homogeneous.

From these remarks it also becomes clear that the first scattering plate58 significantly contributes to the irradiance distribution in the maskplane 70, and for that reason its optical properties should be carefullydesigned.

Generally, the first scattering plate 58 should be designed such thatthe scattering effect is substantially independent from the positionwhere a light beam impinges on the plate. This involves substructuresthat produce the entire angular distribution should be small incomparison to the pitch of the microlenses 561Y, 561X contained in thefirst integrator member 561. Such a relation is considered to befulfilled if the pitch of the substructures is smaller than 20%,preferably smaller than 10%, of the pitch of the corresponding first orsecond microlenses 561Y, 561X.

In principle it is possible to realize the first scattering plate 58 asa conventional glass disc having one or two etched or ground surfaces,for example. Such conventional scattering plates have the advantage thatthe angular distribution varies completely randomly over its surface,which is generally a desired effect for the reasons explained above. Onthe other hand, these conventional scattering plates have thedisadvantage that their optical properties cannot be tailored to thespecific desired properties well enough. For example, it is usually notpossible to obtain a strongly anamorphic angular distribution in the Xand Y directions as is shown in FIG. 14. Instead, conventionalscattering plates produce a very broad angular distribution both in theX and the Y direction. As a result, a significant amount of light has tobe blocked by field stops in order to obtain an illuminated field 14having a high aspect ratio.

For that reason various alternatives are proposed below how the firstscattering plate 58 may be realized such that its optical properties maybe accurately determined by its design. Nevertheless the firstscattering plate 58 shall have certain randomized or carefully selectedregular properties that are involved for preventing undesiredinteractions with the regular features of the optical integrator 56.

4.2 Undesired Interactions Between First Scattering Plate and OpticalIntegrator

Before various embodiment of the scattering plate 58 are described inmore detail, possible interactions between the scattering plate 58 andthe optical integrator 56 will be explained.

4.2.1 Superposition

FIG. 16 is an enlarged cutout of FIG. 13 and schematically shows thelight propagation between substructures 58X of the first scatteringplate 58 and second microlenses 561X of the first integrator member 561.For the sake of simplicity the first microlenses 561Y and a support forthe substructures 58X are not shown because these elements do not havean impact on the angular distribution of the projection light in the Xdirection. The substructures 58X are formed by cylindrical microlenseshaving longitudinal axes that extend along the Y direction. Eachsubstructure 58X produces a divergent light bundle indicated by brokenlines 85. The pitch of the microlenses 561X and the substructures 58X isdenoted by pi_(n) and p_(sc), respectively. In this specific embodimentso that every two microlenses 561X a sequence of 5 substructures 58X isrepeated.

The larger the distance z between the first scattering plate 58 and thesecond microlenses 561X is, the larger is the number of divergent lightbundles 85 that superimpose on the second microlenses 561X. This isillustrated in FIG. 16 by different degrees of shading. As a result ofthis superposition, there is a periodic irradiance variation along the Xdirection. The amount of the fluctuations decreases with growingdistance z because the irradiance of a single light bundle produced by asubstructure 58X decreases with z².

As a result of these fluctuations, also the angular distribution of thelight impinging on the microlenses 561X may be different. For example,the upper two microlenses 561X shown in FIG. 16 are subjected todifferent, although symmetrical, irradiance and angular distributions.As a result of the relationship p_(sc)=2/5^(>)pi_(n), every second oneof the second microlenses 561X is subjected to the same irradiance andangular distribution, and thus every second secondary light source 82will be equal. If only two different types of light sources 82 arepresent in the pupil plane 54, the different irradiance distributions inthe mask plane 70 will not have an averaging effect to such an extentthat a substantially homogeneous irradiance distribution is achieved.

FIG. 17 shows the first scattering plate 58 is not arranged in front of,but behind the first integrator member 561, as is the case in FIGS. 10and 12. From this illustration it becomes clear that reversing thesequence of the second microlenses 561X and the substructures 58X doesnot solve this problem because the irradiance and angular distributionon the first scattering plate 58 is repeated every 5 substructures 58X.Consequently, identical configurations of substructures 58X and secondmicrolenses 561X still repeat quite frequently. Since each identicalconfiguration produces the same secondary light source, theaforementioned averaging effect is small. For example, even with alarger substructure pitches p_(sc)=200 μm, the period with whichidentical configurations repeat is 1 mm.

4.2.2 Talbot Effect

There is another effect that occurs in the case of combinations ofmicrolens arrays in the optical integrator 56 and periodic scatteringsubstructures in the first scattering plate 58. Periodic structures areknown to form exact images of themselves at integer multiples of thedistance through Fresnel diffraction when illuminated by a coherent orpartial coherent wave. This self-imaging phenomenon is called the Talboteffect. In addition, multiple phase-transformed Fresnel images areproduced at fractional-Talbot distances. The Talbot phenomenon indicatesthat in any plane behind a periodic optical element a certainperiodicity is observed.

The Talbot effect manifests itself as significant interference patternshaving a high contrast at certain distances from the periodicstructures. These distances, which are referred to as Talbot distancesz_(n), are given by Z_(n)=n·Z_(T), where Z_(τ)=2p²/λ. Here λ is thewavelength of the incident light, p is the period of the structures andn is a positive integer. However, interference patterns with a smallercontrast are also observed at certain fractional Talbot distances, forexample at 2/9 Z_(τ) or 3/14 Z_(τ). FIG. 18 shows schematically Talbotinterference patterns at Talbot distances Z_(τ)and 2 Z_(T) and also at anumber of fractional Talbot distances.

Since the Talbot effect is based on diffraction, it is most prominent ifthe degree of coherence is close to 100%. The laser light thatilluminates one or more pitches of the first scattering plate 58 isgenerally partially coherent. The degree of coherence of the projectionlight may be estimated on the basis of the speckle contrast that ispresent at each point in the illumination system 12. Usually the specklecontrast is in the range between 10% and 20%. This is sufficient forobserving prominent Talbot interference patterns at Talbot distancesbehind the first scattering plate 58.

High contrast interference patterns may occur at distances z in theorder of 60 or 90 mm measured from the first scattering plate 58. Atthese distances the superposition effect described above in 4.2.1 isnegligible, at least with scattering substructure pitches p_(sc) below100 μm.

If the distance between the first scattering plate 58 and the firstintegrator member 561 is equal or close to a (fractional) Talbotdistance in which high contrast interference patterns occur, Moirepatterns are observed that are a result of the periodic Talbotinterference pattern on the one hand and the periodic arrangement of thefirst and second microlenses 561Y, 561X on the other hand. Although theirradiance distributions produced by each secondary light source 82superimpose in the mask plane 70, these Moire interference patterns maynevertheless introduce non-uniformities of the irradiance distributionin the mask plane 70.

4.3 Different Design Approaches

In the following different approaches will be described that may beemployed to avoid the undesired interactions described above in section4.2.

4.3.1 Distance

For avoiding Moire patterns caused by interactions between Talbotinference patterns and arrays of microlenses contained in the firstintegrator member 561, care should be taken that the distance z betweenthe first scattering plate 58 and the first integrator member 561 doesnot coincide with a Talbot distance or any fractional Talbot distance inwhich high irradiance contrasts are observed. A range of suitabledistances may be determined with the help of simulation programs whichcompute the contrast of Talbot interference patterns in various integeror fractional Talbot distances.

4.3.2 Pitch Selection

FIG. 19 shows, in a further enlarged view similar to FIG. 16, a firstapproach how too frequent identical configurations of substructures 58Xand second microlenses 561X may be avoided. The second microlenses 561Xand the substructures 58X have pitches pi_(n)=500 μm and p_(sc)=47 μm,respectively. 47 is prime to 500 so that the irradiance and angulardistribution produced by the concave microlenses 58X is repeated on thesecond microlenses 561X only after 47·500 μm=23.5 mm.

FIG. 20 shows, for illustrative purposes only, a top view of a firstgrid 561X′ and a second grid 58X¹ having pitches that are selected suchthat over 10 periods of the first grid 561X′ the lines of the secondgrid 58X′ always have a different relative position to a single periodof the first pitch 561X′.

4.3.3 Irregular Substructures in Scattering Plate

Another approach to avoid frequent identical configurations ofsubstructures 58X and second microlenses 561X is to use irregularsubstructures. The irregularity may be in terms of the arrangement ofidentical substructures and/or manifest itself in differentsubstructures. It should be noted that this approach may be combinedwith the pitch selection according to section 4.3.2.

In the embodiments described above the substructures 58X of the firstscattering plate 58 are realized as cylindrical microlenses. However, adivergence in one or two directions may also be produces with the helpof diffractive optical elements. In the following sections variousembodiments of refractive and diffractive designs for the firstscattering plate 58 will be described.

4.3.4 Refractive Designs

FIGS. 21 and 22 show a first scattering plate 158 in a perspective and asectional view along the X direction, respectively. The first scatteringplate 158 includes an array of alternating convex cylindricalmicrolenses 1581 and concave cylindrical microlenses 1582 that bothextend along the Y direction. The scattering plate 158 thus increasesthe divergence only in the X direction. Due to the cylindrical shapewith constant curvature the angular distribution is, at least to a goodapproximation, rectangular in the X direction.

If the divergence shall be increased also in the Y direction, a similararray of microlenses 1581, 1582 may be provided on the other side of thefirst scattering plate 158, but with an orthogonal orientation of themicrolenses. In principle it is also possible to have other thanorthogonal orientations, as will be explained further below in section5.2.1, to have crossed microlenses on one side of the plate, or toprovide separate supports for the each array of microlenses. If thedivergence produced by the first scattering plate 158 shall be smallerin the Y direction than in the X direction, the curvature of themicrolenses producing a divergence in the X direction has to be smallerthan the curvature of the microlenses producing a divergence in the Ydirection.

The microlenses 1581, 1582 may be formed by molding or by machining asubstrate 1557 in a manner that is similar to the manufacture of themicrolenses contained in the optical integrator 56.

Various embodiments of first scattering plates will now be describedwith reference to FIGS. 23 to 32, which show similar to FIG. 22cross-sections along the X direction. As a matter of course, also inthese embodiments a second array of orthogonal microlenses may beprovided on the other side of the support, the same side of the supportor on a different support if the divergence shall be increased in the Ydirection as well. Furthermore, it is also possible to have differentdesigns for microlenses producing a divergence in the X direction andmicrolenses producing a divergence in the Y direction.

FIG. 23 shows a cross-section through a first scattering plate 258 thatincludes only convex cylindrical microlenses 2581 having the same shape.

FIG. 24 shows a cross-section through a first scattering plate 358 thatincludes only concave cylindrical microlenses 3582 having the sameshape.

FIG. 25 shows a cross-section through a first scattering plate 458 thatis similar to the scattering plate 158 shown in FIGS. 21 and 22.However, convex cylindrical microlenses 4581 are separated from eachother by rectangular plane areas 4583 extending along the Y direction.This separation ensures that no sharp edges are present where adjacentmicrolenses 4581 meet. Such edges often have an undesired effect on theangular distribution.

If the plane areas 4583 have a large width w, a significant portion ofthe light traverses effectively a plane parallel plate, which does notincrease the geometrical optical flux. However, the widths w of theareas 4583 are so small that the light is diffracted, similar to what isobserved at an array of small slits. More particularly, the width w isdetermined such that the angular distribution caused by diffraction isat least approximately the same as the angular distribution caused bythe microlenses 4581.

In the first scattering plates 258, 358 shown in FIGS. 23 and 24,respectively, all microlenses have an identical shape and form a regulararray. In order to avoid undesired interaction with the first integratormember 561, the pitch of the microlenses should be carefully selected inaccordance with section 4.3.2 above.

FIG. 26 shows a cross-section through a first scattering plate 558 thatincludes a plurality of different concave cylindrical microlenses 5582.The microlenses 5582 have identical curvatures but different pitches p₁,p₂, . . . , p_(n). The longitudinal edges formed between adjacentmicrolenses 5582 are arranged in a plane 5585 which is parallel to abase plane of the first scattering plate 558.

The microlenses 5582 produce, as a result of their varying pitches p₁,p₂, . . . , p_(n), angular distributions having an approximatelyrectangular shape, but with varying widths. If the pitches p₁, p₂, . . ., p_(n), of the microlenses 5582 vary according to a Gaussianprobability distribution, the overall angular distribution resultingfrom the contributions of all microlenses 5582 will have an at leastapproximately Gaussian shape, too. This will be explained in more detailfurther below with reference to FIG. 45.

If the pitches p₁, p₂, . . . , p_(n) vary within a small range, forexample between 48 μm and 50 μm, the deviations from a rectangularangular distribution are small. Even small variations of the pitches p₁,p₂, . . . , p_(n) suffice to introduce a pseudo-random irregularity thatreduces undesired interactions between the first scattering plate andthe first integrator member 561.

By carefully selecting the height of the center of curvatures, it ispossible to influence also diffractive effects that become present ifthe pitch p₁, p₂, . . . , p_(n) is very small, for example smaller than50 μm at a wavelength λ=193 nm. In such configurations the scatteringfunction of the first scattering plate 558 is therefore a combination ofrefractive and diffractive effects that may both be selectivelydetermined by selecting the aforementioned design parameters.

Since the array of microlenses 5582 is not strictly periodic, it doesnot produce significant Talbot interference patterns, or the contrast ofthe Talbot interference patterns is significantly reduced. This resultsin a more homogenous intensity distribution in the mask plane 70.

FIG. 27 shows a cross-section through another first scattering plate 658that also includes concave microlenses 6582 having varying pitches p₁,p₂, . . . , p_(n)—In contrast to t FIG. 26, the vertex lines of themicrolenses 6582 and not the longitudinal edges between adjacentmicrolenses are arranged in a common plane 6685 that is parallel to abase plane of the first scattering plate 658. This has the effect thatthe longitudinal edges between adjacent microlenses 6582 are arranged atdifferent heights from the base plane, and thus the microlenses 6582 aregenerally not symmetrically shaped with respect to their vertex lines.As a result, the microlenses 6582 produce asymmetrical angulardistributions. However, if the number of microlenses 6582 issufficiently large, a highly symmetrical angular distribution willnevertheless be obtained.

The pitch variation has the effect of reducing undesired interactionsbetween the first scattering plate and the first integrator member 561,and in particular of reducing the contrast of Talbot interferencepatterns.

FIG. 28 shows a section through a first scattering plate 758 that alsoincludes concave microlenses 7582 having varying pitches p₁, p₂, . . . ,p_(n). In contrast to FIGS. 26 and 27, however, neither the vertices ofthe microlenses 7582 nor the longitudinal edges between adjacentmicrolenses 7582 are arranged in a common plane. This further increasesthe pseudo-random irregularity of the scattering plate 758, which has anadvantageous effect in view of undesired interactions with the firstintegrator member 561.

The pseudo-random irregularity may be still further increased byproviding microlenses 7582′ having varying widths along theirlongitudinal axes. FIG. 29 shows a top view on a first scattering plate758′ that exploits this principle. Here every second edge 7587′ betweenadjacent microlenses 7582′ is curved in a pseudo-randomly manner so thatthe pitch of each microlens 7582′ varies in the Y direction. Thisprinciple may be employed in any of FIGS. 21 to 28. In the scatteringplate 758′ the edges 7587′ have all, in the top view shown, the sameshape. However, even this shape may be different for each microlens7582′. Of course it is also possible to have curved edges 7587′ betweeneach arbitrary pair of microlenses 7582′.

FIG. 30 shows a cross-section through a scattering plate 858 including aplurality of convex cylindrical microlenses 8581. All microlenses 8581have the same pitch p, but the curved surfaces of the microlenses 8581have different non-circular cross-sections. For illustrative reasons thedifferences are exaggerated in FIG. 30. In order to introduce anirregularity in the microlens array, smaller differences between thecurved surfaces of the microlenses 8581 may suffice.

Similar to shown in FIGS. 26 to 29, the angular distributions producedby the first scattering plate 858 are not perfectly rectangular, buthave slopes at the edges. However, using cylindrical microlenses havinga non-circular cross-section considerably enlarges the design freedom.By carefully designing the curved surfaces of the microlenses 8581 it ispossible to produce almost any arbitrary angular distribution, toproduce desired non-uniformities in the irradiance distribution in themask plane 70, or to compensate for effects that would otherwise produceundesired non-uniformities in the irradiance distribution in the maskplane 70.

FIG. 31 shows a cross-section through a first scattering plate 958including a plurality of convex cylindrical microlenses 9581. The firstscattering plate 958 differs from FIG. 30 in that the microlenses 9581have different curved surfaces, too, but all these surfaces havecircular cross-sections with different radii r₁, r₂, . . . , r_(n).

In view of obtaining an approximately rectangular angular distribution,it may be advantageous in both embodiments shown in FIGS. 30 and 31 tocombine the surface shape variation with a pitch variation that has beenexplained above in connection with the embodiments shown in FIGS. 26 to28.

FIG. 32 shows a section through a first scattering plate 1058 that has acompletely randomized surface. Such a surface may be obtained withcertain manufacturing processes that involve stochastic process steps.For example, by grinding and/or etching glass screens the surface shapeobtained in this process cannot be controlled up to the very detail, andit will therefore vary randomly, at least within certain limits.However, the angular distribution generated by such a completely randomsurface is always at least substantially Gaussian, which restricts theuse of such a first scattering plate 1058 to applications in which aGaussian distribution is desired. Furthermore, the parameters of theGaussian distribution are often difficult to control in themanufacturing process.

Therefore it is also envisaged to use microlithographic methods toproduce a two-dimensional pseudo-random surface that produces a Gaussianangular distribution as they are produced by random surfaces obtainedwith manufacturing processes involving stochastic process steps. Theadvantage of such surfaces is that the parameters of the Gaussiandistribution can be exactly predicted so that all manufacturedscattering plate have identical optical properties.

FIGS. 33 and 34 show a first scattering plate 1158 in a perspective viewand a section along line XXXIV-XXXIV, respectively. The first scatteringplate 1158 includes a plurality of microlenses 11581 each having a toricshape.

The curvature of the toric microlenses 11581 is, in the embodimentshown, larger in the X-Z plane than in the Y-Z plane. This ensures thatthe divergence produced in the X direction is greater than thedivergence produced in the Y direction. With the use of toricmicrolenses 11581 there is no need to provide microlenses on both sidesof the scattering plate if a divergence shall be produced both in the Xand in the Y direction.

FIGS. 35 and 36 show a first scattering plate 1258 in a top view and asectional view along line XXXVI-XXXVI. The first scattering plate 1258includes a plurality of convex spherical microlenses 12581 that arearranged in a regular grid-like array. Depending on the desired angulardistribution embodiments with aspherical microlenses may be used. Eachmicrolens 12581 has a quadratic circumference so that the optical effectof the microlenses 12581 is not completely rotationally symmetric.Instead, the angular distribution has a fourfold symmetry. The firstscattering plate 1258 is suitable only for those applications in which amore or less rotationally symmetric angular distribution is desired.However, such a design is particularly advantageous for the secondscattering plate 60, as will be explained further below.

4.3.5 Diffractive Designs

In the following various embodiments will be described with reference toFIGS. 37 to 43 in which the first scattering plate 58 includesdiffractive optical structures. These diffractive structures increasethe divergence in at least one direction. In the following a group ofdiffractive structures that produces a substantially complete angulardistribution will be referred to as diffractive cell. A singlediffractive cell therefore corresponds to a microlens of the refractivedesigns described in section 4.3.4.

Diffractive scattering plates make it possible to produce almost anyarbitrary angular distribution. However, the angular distributionproduced by a diffractive cell is always discrete, whereas the angulardistributions produced by smoothly curved refractive surface iscontinuous. The smaller the cell is, the more discrete is the producedangular distribution, and vice versa.

FIG. 37 shows a top view of a diffractive cell M1 containing a pluralityof diffractive structures 92. This type of diffractive cell is oftenreferred to as computer generated hologram (CGH) and produces apredefined angular distribution in at least one direction.

FIG. 38 a shows another cell M2 containing diffractive structures 93that form a Fresnel lens that is, at least substantially, rotationallysymmetric. FIG. 39 a shows a top view on a diffractive cell M3containing diffractive structures 94 that form a cylindrical Fresnellens.

If diffractive cells M are arranged in a strictly periodic array, theundesired interactions between the first scattering plate 58 and theoptical integrator 56 may arise that have been explained above insection 4.2. For that reason the array of the diffractive cells M shouldbe randomized, at least to a certain extent, and/or a suitable pitchselection as explained in section 4.3.2 should be made.

FIG. 40 shows a schematic top view of a first scattering plate 1358including a plurality of diffractive cells M that are arranged in aperiodic grid-like manner. It is assumed that the diffractive cells Mscatter light only in the X direction. In this direction the pitch p ofthe diffractive cells M should be selected in accordance with theprinciples explained in section 4.3.2 above in order to avoid frequentcorrelations between the diffractive cells M on the one hand and themicrolenses of the first integrator member 561 on the other hand.

FIG. 41 shows a schematic top view of a first scattering plate 1458including a plurality of diffractive cells M. In this embodiment thepitch p of the diffractive cells M varies along the X direction in whichthe divergence is increased. The effect of smaller diffractive cells Mdepends on the kind of diffractive structures contained therein. Forexample, if the diffractive cell M3 shown in FIG. 39 a is reduced in itslength without altering the arrangement of the diffractive structures94, it will produce a smaller angular distribution. If the length in theX direction of the diffractive cell M1 shown in FIG. 37 is reduced, theangular distribution will have the same width, but the distribution willbecome more discrete.

FIG. 42 shows a schematic top view of a first scattering plate 1558 inwhich the pitch of the diffractive cells M varies differently in eachrow. This further increases the pseudo-random irregularity of theangular distribution produced by the entirety of diffractive cells M.

FIG. 43 shows in a schematic top view a first scattering plate 1658 thatincludes a plurality of diffractive cells M₁, M₂, . . . , M₆ havingequal pitches. Thus the cells M₁, M₂, . . . , M₆ are arranged in aregular manner similar to the embodiment shown in FIG. 40. However, inthis embodiment the diffractive cells, M₁, M₂, . . . , M₆ differ fromeach other as far as the arrangement of diffractive structures containedtherein is concerned. This is comparable to the refractive scatteringplates 858 and 958 shown in FIGS. 30 and 31, respectively.

The different cell structures may be obtained by scaling up or down agiven cell structure. This corresponds to an increase or decrease of theradii ri in the refractive scattering plate 958 shown in FIG. 31. For adiffractive structure an example of this scaling transformation is shownin FIG. 38 b. In the diffractive cell M2′ the diffractive structures 93′are obtained by scaling up the diffractive structures 93 of thediffractive cell M2 shown in FIG. 38 a.

Another approach to obtain different cell structures is to shift a givencell structure along the direction in which a scattering effect shall beachieved. This is exemplarily illustrated in FIG. 39 b. Here thediffractive cell M3′ is obtained from the diffractive cell M3 shown inFIG. 39 a by shifting the diffractive structures 94 along the Xdirection. This is a similar effect as it is achieved in the refractivescattering plate 658 shown in FIG. 27.

It should be noted that the proposed variations with respect to cellpitch and cell structure will usually affect the angular distribution.However, this may be taken into account in the design of the diffractivecells M so that a desired angular distribution is obtained with apseudo-randomized array of diffractive cells.

As a matter of course, some or all variations, in particular withrespect to the cell pitch p and the cell contents, may be combined tofurther increase the random nature of the first scattering plate 58which avoids undesirable interactions with the first integrator member561.

5 Second Scattering Plate

In the following the general function and various embodiments for thesecond scattering plate 60 will be explained in more detail.

5.1 General Function of Second Scattering Plate

The second scattering plate 60 may have one or more of the followingfunctions:

One function of the second scattering plate 60 may be to ensure that theirradiance distribution in the mask plane 70 along the Y direction hasthe desired shape. This can involve adapting the angular distribution,which the second scattering plate 60 as such produces along the Ydirection, to the angular distributions along this direction produced bythe first scattering plate 58 (if any) and the optical integrator 56.

If the irradiance distribution along the Y direction (i.e. scandirection) is rectangular, undesired feature size variations may occuras a result of the pulse-quantization. For reducing or even completelyavoiding the pulse-quantization effect, which is described in moredetail in International Application WO 2005/078522 mentioned above, theirradiance should smoothly increase and decrease at both ends of theirradiance distribution. The slope may be linear, which results in anoverall trapezoidal shape of the irradiance distribution, or may have asubstantially Gaussian shape, for example.

Another function of the second scattering plate 60 may be to avoidundesired correlations between the light bundles produced by thesecondary light sources 82. This implies that adverse effects caused bydiffraction in the second integrator member 562 on the irradiancedistribution are reduced.

A still further function of the second scattering plate 60 may be toimprove the angular distribution of the light as it traverses the maskplane 70. To this end it is preferred to arrange the second scatteringplate 60 between the optical integrator 56 and the condenser 62. In thisposition the second scattering plate 60 is arranged at some distancefrom the pupil plane 54 so that a blurring effect for the secondarylight sources can be achieved. Preferably the secondary light sourcesare enlarged by the blurring effect to such an extent that adjacentsecondary light sources about or even overlap in the pupil plane 54. Asa result, it is possible to obtain a continuous angular distribution inthe mask plane 70 that may be advantageous for certain illuminationsettings.

The second scattering plate 60 may further have an advantageous effecton the telecentricity and ellipticity properties of the illuminationsystem 12.

Similarly to the first scattering plate 58, the second scattering plate60 should have the property that the dimensions of the substructuresproducing the angular distribution is small, preferably smaller than 20%of the pitch of the microlenses of the optical integrator 56.

In the following it is assumed that the desired irradiance distributionalong the Y direction has a Gaussian shape with a half value width thatensures the desired aspect ratio of the illuminated field 14. As hasbeen mentioned before, such a shape of the irradiance distribution isadvantageous in view of a reduction of the pulse quantization effect. Itis possible to produce such a irradiance distribution mainly with acombination of the optical integrator 56 and the scattering plates 58,60. This means that there is no need to block out light, for exampleusing gradient absorption filter elements. Possible realizations how aGaussian irradiance distribution along the Y direction may be obtainedwill be explained below with reference to the embodiments described insection 5.2.

Enlarging the secondary light sources both in the X and the Y directioninvolves, however, that the second scattering plate 60 also increasesthe divergence in the X direction. This is as such undesirable becauseit results in a non-rectangular irradiance distribution in the Xdirection, i.e. perpendicular to the scan direction. Smooth slopes atthe lateral edges of the irradiance distribution along the X directionhave to be blocked out, for example using the field stop 66. If thelight losses shall be kept small, the second scattering plate 60 has tohave an anamorphic scattering effect similar to the first scatteringplate 58. Since such a second scattering plate 60 does not enlarge thesecondary light sources in the X direction, a tradeoff has to be foundbetween having a substantially continuous angular distribution in themask plane 70 on the one hand and small light losses on the other hand.

Here it is assumed that the secondary light sources shall be increasedboth in the X and Y direction. To this end the second scattering plate60 produces an angular distribution that is rotationally symmetrical andhas a Gaussian shape, as is illustrated in FIG. 44.

5.2 Different Design Approaches

In the following different design approaches for the second scatteringplate 60 will be explained with reference to FIGS. 45 to 49.

In principle the second scattering plate may be realized using arefractive design, a diffractive design or a design combining refractiveand diffractive effects. For that reason all designs that have beendescribed above in section 4.3.1 in connection with the first scatteringplate 58 may equally be used for the second scattering plate 60.However, refractive designs are generally more preferred for the secondscattering plate 60. This is because diffractive optical elementsusually cause, as a result of their limited diffraction efficiency,higher light losses than refractive optical elements. The followingremarks relate to refractive designs, but they also apply to diffractiveremarks if the microlenses are replaced by appropriate diffractivecells.

If a two-dimensional angular distribution shall be produced as shown inFIG. 44, the following approaches should be contemplated:

5.2.1 Two Microlens Arrays on Different Sides

A two-dimensional angular distribution can be obtained, as has beenexplained further above, by arranging a first array of parallelmicrolenses on one side of a substrate and a second array ofperpendicular microlenses on the other side. Alternatively, the arraymay be formed on two distinct substrates. In both cases it is possibleto determine the scattering effect for each direction completelyindependent from one another by carefully selecting the design parameterof each array.

For generating a Gaussian angular distribution an approximation may beused that will now be explained with reference to FIG. 45. Here thesecond scattering plate 60 includes a plurality of microlenses thatproduce rectangular angular distributions of different angular widths.The angular widths vary with a Gaussian probability distribution arounda center angle α_(o)=0°. The superposition of all rectangular angulardistributions having different widths then results in an overall angulardistribution having a Gaussian shape. This is illustrated in FIG. 45 forfour different rectangular angular distributions AD1, AD2, AD3 and AD4.The larger the number of microlenses is, the better is the approximationto a Gaussian angular distribution.

In order to further smooth the stepped profile shown in FIG. 45 alongthe scan direction, it may be advantageous to deviate from theorthogonal orientation of the two arrays of microlenses. For example,the microlenses on both sides may form an angle between 89° and 80°.

If the arrays of microlenses are arranged on different substrates, thesubstrates may be arranged such that one or both substrates can berotated around an axis coaxial or at least parallel to the optical axis26 with the help of a manipulator. Then it is possible to adjust theangle between the microlens arrays.

A deviation of the two microlens array orientations from 90° has theeffect that one array produces an angular distribution having a portionalong the direction of the other array. This portions results in asmoothing effect. Mathematically speaking, the result is a convolutionof the stepped profile as shown in FIG. 45 with a projection of thisprofile. The actual width of the projection depends on the angle betweenthe two microlens array orientations and it is proportional to thecosine of this angle. Thus, if the orientation angle is chosen such thatthe profile projection width is comparable to the step width, theconvolution has a considerable smoothing effect.

It may also be considered to arrange the microlens arrays such that theyare not aligned parallel to the scan direction. If the microlensesproduce angular distributions having small ripples, and these ripplesare aligned parallel to the scan direction, the total light energy(dose) impinging on a single point on the mask will vary accordingly.If, however, no microlens array is aligned parallel to the scandirection, the ripples in the irradiance distribution are also inclinedwith respect to the scan direction. The scanning motion then results inan averaging effect over many ripples, which has the effect of aconstant total light energy (dose) received by each point on the mask.

Apart from that, a configuration in which no array is aligned parallelto the scan direction has the advantage that undesired Moire patternsare reduced that may otherwise occur as an interaction with the regularmicrolens arrays of the optical integrator 56.

If both arrays are arranged on one substrate, this may be rotatedaccordingly in order to avoid a parallel orientation of a microlensarray with respect to the scan direction. If both arrays are arranged ondifferent substrates, it may suffice to rotate only one substrate. Inorder to maintain the relative angle between the arrays, however, bothsubstrates may be commonly rotated.

If no adjustment of the angular positions of the microlens arrays isdesired, there is no need for manipulators. In these cases the substrate(s) may be fixedly received in mounts that ensure the desired angularpositions or the microlens arrays.

5.2.2 Two Crossed Microlens Arrays on One Side

If two arrays of cylindrical microlenses are crossed on one side of asubstrate, this results in configurations similar to what has beendescribed above for the first scattering plate 1158 with reference toFIGS. 33 to 34. The microlenses 11581 shown in this embodiment have atoric surface, but a surface obtained by crossing two cylindricalsurfaces may also be used.

As a matter of course, also in this case a non-orthogonal orientation ofthe cylindrical lenses may be considered.

5.2.3 Rotationally Symmetric Profile

As a further alternative, rotationally symmetrical microlenses such asshown in FIGS. 35 and 36 may be used. Also in this embodiment the radiiof the microlenses should vary according to the Gaussian probabilitydistribution in order to obtain the Gaussian overall angulardistribution shown in FIGS. 44 and 45. Other lens parameters, e.g.center of curvature or refractive index, may additionally oralternatively be varied.

5.3 Other Design Aspects

In the following other advantageous design aspects for the secondscattering plate 60 are described with reference to FIGS. 46 to 49.

FIG. 46 shows, in a highly schematic representation that is not toscale, three third microlenses 562X, the condenser 62 and the field stopplane 64. Two groups of broken lines 97, 98 indicate ray bundles eachleaving the third microlenses 562X under the same aperture angle andconsequently converge to the same field point in the field stop plane64. The higher the number of third microlenses 562X is, the more rays97, 98 will contribute to the irradiance in the field stop plane 64under different angles. However, there will not be a completelycontinuous angle distribution in the field stop plane 64 due to therestricted number of third microlenses 562X. The same applies, ofcourse, for the Y direction.

FIG. A1 shows the same configuration, but now with the additional secondscattering plate 60 arranged between the optical integrator 56 and thecondenser 62. The second scattering plate 60 produces a continuousangular distribution which is indicated in FIG. 47 by a plurality ofscattered light rays 99. Preferably the maximum scattering angle α_(max)is determined such that scattered light rays 99′ having a maximumscattering angle ot_(max) would, if extended backwards towards the thirdmicrolenses 562X, impinge on the third microlenses 562X at a distance δthat is at least as large as the pitch p of the third microlenses 562X.

If looked at from the field stop plane 64, it then seems that theprojection light impinging on the field stop plane 64 is produced bysecondary light sources 82′ that have an extension in the X directionwhich is at least as large as the pitch p of the third microlenses 562X.In other words, the secondary light sources then abut or even overlap inthe pupil plane 54 in the X direction. Of course the same considerationsapply also for the Y direction.

As a result of the abutting or overlapping secondary light sources, theprojection light impinges on any point in the field stop plane 64 with acontinuous range of illumination angles, wherein the range is determinedby the illumination setting.

Ideally the irradiance distribution in the pupil plane 54 ishomogeneous. This property may be achieved with the help of the secondscattering plate as well, as will be explained with reference to FIGS.48 and 49.

FIG. 48 shows a graph in which the intensity distribution J in the pupilplane 54 is plotted against the Y direction for three adjacent secondarylight sources 82. In the schematic representation the secondary lightsources 82 are, for the sake of simplicity, characterized by trapezoidalintensity distributions. Between these distributions gaps remain throughwhich no light passes.

However, by carefully designing the scattering properties of the secondscattering plate 60 it is possible to effectively broaden the secondarylight sources 82 such that the half value widths of the intensitydistribution of each single secondary light source 82 meet.

What this means is shown in FIG. 49. Here it is, again for the sake ofsimplicity, assumed that the second scattering plate 60 effectivelybroadens the intensity distributions of the secondary light sources 82in the pupil plane, but retains their trapezoidal shape. The intensitydistributions are broadened to such an extent that the half value widthsw of adjacent intensity distributions 82′ abut. The overlappingintensity distributions 82′ then homogeneously illuminate the pupilplane 54, and all illumination angles are present with the sameintensity between 0° and the maximum angle α_(max) which is determinedby the diameter of the pupil. Of course this holds true, in a strictsense, only in the case of a conventional illumination setting withmaximum σ. In the case of other illumination settings, the completelyand homogeneously illuminated areas are defined by the setting.

Therefore the angular distribution may be defined solely with thosemembers that are provided for achieving different illumination settings.In the illumination system 12 these include the first optical rasterelement 34, the zoom lens group 46 and the pair 48 of axicon elements.The optical integrator 56 and the scattering plates 58, 60 thus ensurethat no other parameters have to be considered in defining the angulardistribution in the mask plane 70 other than the intensity distributionin the pupil plane 54 defined by those members.

If the illumination system 12 includes three optical elements thatincrease the geometrical optical flux, namely the optical integrator 56and the two scattering plates 58, 60, it has to be considered how theincrease is distributed among these three optical elements. In thisrespect it has been found advantageous to define the maximum divergenceproduced by the first scattering plate 58, the optical integrator 56 andthe second scattering plate 60 in the following way:

NA1X≦NA2X,

NA2X>5·NA2Y,

0.9·NA3Y<NA3X<1.1·NA3Y.

where NA1X is the maximum divergence angle produced by the firstscattering plate 58, NA2X and NA2Y are the maximum divergence anglesproduced by the optical integrator 56 and NA3X, NA3Y are the maximumdivergence angles produced by the second scattering plate 60 for the Xand Y directions, respectively.

1.-27. (canceled)
 28. An optical system, comprising: an opticalintegrator configured so that, when light is incident thereon, theoptical integrator produces secondary light sources; a first scatteringstructure arranged, along a light propagation direction of the system,in front of the optical integrator, the first scattering structurecomprising first subelements configured to introduces a divergence ofincident light only in one direction; and a second scattering structurearranged, along a light propagation direction of the system, behind theoptical integrator, the second scattering structure comprising secondsubelements configured to introduce a divergence of incident light intwo directions, wherein the system is a microlithographic illuminationsystem.
 29. The optical system of claim 28, wherein: the opticalintegrator comprises a fly-eye integrator; the fly-eye integratorcomprises first and second integrator members; and each of the first andsecond integrator members comprises a plurality of focusing opticalsubelements.
 30. The optical system of claim 28, wherein, in a directionperpendicular to an optical axis of the optical system, the firstoptical subelements have a first pitch and the second opticalsubelements have a second pitch different from the first pitch.
 31. Theoptical system of claim 30, wherein the first pitch is prime to thesecond pitch.
 32. The optical system of claim 28, wherein the secondoptical subelements are arranged in a non-periodic array.
 33. Theoptical system of claim 28, wherein each of the second opticalsubelements comprises a microlens.
 34. The optical system of claim 33,wherein at least some of the microlenses have a cylindrical shape. 35.The optical system of claim 28, wherein each of the second opticalsubelements is configured to produce an anamorphic angular lightdistribution during use of the optical system so that the divergence isintroduced to different extents in orthogonal directions.
 36. Theoptical system of claim 28, wherein the first optical subelements haveshapes defined by first borderlines, the second optical subelements haveshapes defined by second borderlines, the first and second borderlinesdefine an angle which is greater than 0.1° and less than 89.9°.
 37. Theoptical system of claim 28, wherein the second scattering structurecomprises first and second kinds of second optical subelements, thefirst kind of optical subelements are configured to introduce thedivergence in a first direction during use of the optical system, andthe second kind of optical subelements are configured to introduce thedivergence in a second direction different from the first direction. 38.The optical system of claim 37, wherein: the optical system is containedin a microlithographic projection exposure apparatus which has a scandirection; and the first kind of optical subelements are configured sothat, during use of the optical system, the first kind of opticalsubelements produce a Gaussian angular light distribution in the scandirection of the microlithographic projection exposure apparatus. 39.The optical system of claim 38, wherein the Gaussian angular lightdistribution is approximated by a superposition of a plurality ofsubstantially rectangular angular distributions of different width. 40.The optical system of claim 39, wherein the second type of opticalsubelements are cylindrical microlenses having different pitches. 41.The optical system of claim 28, wherein the first and second scatteringstructures are configured to modify the angular distribution during useof the optical system in a way that would be obtained, in the absence ofthe at least one scattering structure, by enlarging the secondary lightsources such that they abut or overlap.
 42. An apparatus, comprising: anoptical system according to claim 28, wherein the apparatus ismicrolithographic projection exposure apparatus.
 43. The apparatus ofclaim 42, wherein: the apparatus has a scan direction; and the secondkind of optical subelements are configured so that, during use of theoptical system, the second kind of optical subelements produce arectangular angular light distribution perpendicular to the scandirection of the microlithographic projection exposure apparatus. 44.The apparatus of claim 42, wherein the first and second kinds of opticalsubelements, respectively, are on opposite sides of a common support.45. An optical system, comprising: an optical integrator comprising aplurality of focusing optical subelements; a first scattering structurearranged, along a light propagation direction of the system, in front ofthe optical integrator, wherein the first scattering structure isconfigured to increase a geometrical optical flux only in one direction;and a second scattering structure arranged, along a light propagationdirection of the system, behind the optical integrator wherein thesecond scattering structure is configured to increase the geometricaloptical flux in two directions, wherein the system is amicrolithographic illumination system.
 46. The optical system of claim45, wherein the two directions are orthogonal.
 47. The optical system ofclaim 45, wherein the optical integrator comprises a fly-eye integratorcomprising first and second integrator members, and each of the firstand second integrator members comprises a plurality of focusing opticalsubelements, and wherein the first scattering structure is configured toincrease the divergence to such an extent that light, which passesthrough a focusing optical element of the first integrator member, isdistributed over the entire surface of a focusing optical element of thesecond integrator member.
 48. The optical system of claim 45, whereinthe optical system is contained in a microlithographic projectionexposure apparatus which has a scan direction, and wherein the onedirection, in which the first scattering structure increases thegeometrical optical flux, is perpendicular to the scan direction and toan optical axis of the optical system.
 49. An optical system,comprising: a device comprising a plurality of optical subelementsarranged at least substantially in a plane; a first scattering structurearranged, along a light propagation direction of the system, in front ofthe device, the first scattering structure configured to scatter only inone direction; and a second scattering structure arranged, along a lightpropagation direction of the system, behind the device, the secondscattering structure configured to scatter in two directions; whereinthe system is a microlithographic illumination system.